The present invention relates to an apparatus and method for filling initially open molds with non free-flowing composite mixtures and is particularly suitable for the preparation of frictional components such as transmission bands, brake shoes and brake blocks, clutches and similar articles utilizing asbestos.
Such frictional components have customarily been prepared by hand filling a mold, the area of which may be greater than the desired articles, with a composite mixture which includes asbestos fibers, a suitable resin and other powdered components. A given weight of mixture is carefully distributed over the entire area of the mold so as to obtain a layer of uniform thickness and density. The mixture is then compacted under pressure, subsequently cured in an oven and ultimately cut into specific shapes for an intended purpose.
One of the principle problems resulting from this procedure involves the non-uniform thickness and non-uniform density of the sheet of frictional material and consequently, the smaller frictional components. Variances in thickness are attributable to the non free-flowing nature of the mixture which make it difficult to spread the material evenly over the entire surface of the mold. Levelling is usually accomplished by sight and with the aid of brushes and various flat instruments which the operator uses to spread the mixture. However, even though the surface appears to be level, such procedures usually do not produce a uniform density throughout the thickness of the charge of material distributed in the mold.
Were the composite mixture entirely powdered, control over the thickness might be possible, however, the mineral asbestos is added in its fibrous state, each fiber inherently possessing a C-shape. These fibers have a natural tendency to link with one another forming small agglomerations which are not readily separable and which therefore form into small lumps and rolls throughout the mixture as it is being dispersed over the mold. When such a mixture is subjected to pressure sufficient to compress its volume to approximately 20 percent of its original thickness, those areas in which the material thickness had been greater become more firm and hard than the surrounding areas which had been lower, initially. Furthermore, those areas in which the density had been non-uniform are preserved subsequent to compression.
One method of compensation for the non-uniform thickness is to add more of the composite mixture to each mold and then grind away the upper and lower surfaces of the frictional sheet, subsequent to curing, to produce a sheet of uniform thickness. Such a procedure is obviously wasteful of material, for example a thickness of a sheet produced in this manner, having a maximum cured thickness of 0.120 inches will have a thickness of 0.045 inches subsequent to grinding to eliminate waviness on the surface.
Although the final article, pressed, cured and ground, may have a uniform thickness as measured, the wear rate of the article during use is uneven, as obviously the original low areas or "soft spots" are more readily worn away than the areas of greater density. And, while it is desirable for the entire frictional surface to engage mating surfaces during performance, it is the original high, now column-like, areas which support or transfer a significant amount of the force being applied, this having the effect of decreasing the total functioning surface area of the friction component. Thus, some components may be cut entirely from soft spots and will wear out quickly, others may be cut from areas of greater density and will have a greater than normal life, and still others may have combinations of both. Such variations in density in the final sheet of frictional material may be observed by the skilled operator by feel, e.g., flexing and bending the sheet, or by X-ray techniques. Unfortunately, such procedures do not eliminate the problem or the resulting waste.
In order to eliminate problems pertaining to thickness, various devices for mechanically filling the mold have been devised. These are generally related by being designed to spread a layer or layers of the composite mixture from a hopper or similar container. The width of the layer as dispensed is usually equal to the width of the mold and then either the mold or hopper traverses a distance equal to the length of the mold. Notwithstanding the utility of the concept, such apparatus may be operable with powders, but cannot evenly dispense a mixture containing asbestos fibers inasmuch as the fibers agglomerate within the hopper forming bridges above the dispensing end and consequently, areas of little or no material in the mold.
Modification of such apparatus may be made to impart more of a shaking to the mixture in an attempt to eliminate bridging and agglomeration. However, mixtures employed for frictional articles can actually be shaken out of admixture. Thus, it has been found that too violent a treatment will cause the heavier particles in such a composite mixture, e.g., copper, to separate out and fall into the mold prior to the movement of the components of lower density. Furthermore, uneven flow through the hopper often produces voids which are ultimately covered, producing areas of lower density which areas then form "soft spots" in the final article. Obviously, articles of non-uniform density will possess undesirable wear characteristics.
Prior art directed toward devices for dispensing solid particles extends to an early reissue U.S. Pat.. No. 7,555, which discloses reciprocating slides for removing lumps in the hopper and U.S. Pat. No. 773,616 for a fertilizer sower. More recent patents include, U.S. Pat. Nos. 2,689,597; 2,683,619; 3,487,508; and, 3,599,283 which disclose various means for distributing materials into molds or similar articles. Although the foregoing patents are addressed to the problems of uniform distribution, none has been found suitable for a composite mixture usable in the manufacture of frictional components, as evidenced by the fact that preferred production manufacturing techniques still involve manual distribution.
In addition to the purely mechanical problems attendant distribution of asbestos containing mixtures, it is known that asbestos fibers are injurious to health when inhaled and therefore, the minimization of close contact with the material, such as occurs during spreading and levelling of a charge manually, would be highly beneficial to the operator.